Rapidly Evolving Transients in Archival ZTF Public Alerts

TL;DR

This study searches archival ZTF data for rapidly evolving transients, discovering 19 candidates over three years, including Galactic, extragalactic, and potential kilonova-like events, providing insights into their rates and contaminants.

Contribution

First systematic search for rapidly evolving transients in ZTF archival data, identifying new candidates and setting upper limits on kilonova-like event rates.

Findings

19 RET candidates discovered between 2018-2021

8 Galactic and 4 confirmed extragalactic RETs identified

Upper limit on GW170817-like kilonova rate: 2400 Gpc$^{-3}$ yr$^{-1}$

Abstract

We search the archival Zwicky Transient Facility public survey for rapidly evolving transient (RET) candidates based on well-defined criteria between 2018 May and 2021 December. The search yielded 19 bona-fide RET candidates, corresponding to a discovery rate of $\sim 5.2$ events per year. Even with a Galactic latitude cut of $20^\circ$, 8 of the 19 events ($\sim 42$%) are Galactic, including one with a light-curve shape closely resembling that of the GW170817 kilonova (KN). An additional event is a nova in M31. Four out of the 19 events ($\sim 21$%) are confirmed extragalactic RETs (one confirmed here for the first time) and the origin of 6 additional events cannot be determined. We did not find any extragalactic events resembling the GW170817 KN, from which we obtain an upper limit on the volumetric rate of GW170817-like KNe of $R \le$ 2400 Gpc$^{-3}$ yr$^{-1}$ (95% confidence). These results can be used for quantifying contaminants to RET searches in transient alert streams, specifically when searching for kilonovae independently of gravitational-wave and gamma-ray-burst triggers.